The performance of magnetic devices using a stacked structure of magnetic material layers, in particular the performance of magnetic heads, has dramatically improved. In particular, in the technical field of magnetic heads using spin-valve films (Spin-Valve: SV film) there has been great progress.
A “spin-valve film” is a stacked film in which a nonmagnetic layer is sandwiched between two ferromagnetic layers, and one ferromagnetic layer (referred to as a “pinned layer”) is a layer whose magnetization direction is pinned by an antiferromagnetic layer, and the other ferromagnetic layer is a layer whose magnetization can respond to external magnetic fields (referred to as a “free layer”).
The spin-valve film functions as a type of variable resistance device. The resistance of carriers that move through the spin-valve film depends on the carrier spin state. Therefore, by changing the spin state of the spin-valve film with an external magnetic field, it is possible to change the resistance state of the spin-valve film.
The magneto-resistance effect (the MR effect) in which electrical resistance is varied by an external magnetic field gives rise to many physical phenomena. The most well-known are giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR).
The electrical resistance state of a magneto-resistive effect device that includes a spin-valve film is determined by adjacent ferromagnetic layers, for example by the relative relationship between magnetization directions of the pinned layer and the free layer. Typically, in a spin-valve film, when the magnetization directions of the two ferromagnetic layers are aligned parallel, the electrical resistance state is in the “low resistance state”. This state is conventionally represented as the “0” state. On the other hand, when the magnetization directions of the two ferromagnetic layers are aligned antiparallel, the electrical resistance state is in the “high resistance state”. This state is conventionally represented as the “1” state. When the angle between the magnetization directions of the adjacent layers is an intermediate angle, the resistance state is intermediate. A magneto-resistive effect device that uses this phenomenon is widely used in reading heads for HDDs.
A magneto-resistive effect device with two free layers and that does not have a pinned layer and a pinning layer is being investigated as a head suitable for narrow gaps that are suitable for high densification, in contrast to magneto-resistive effect devices having a conventional pinned layer. In this structure, top and bottom magnetic layers with a spacer layer disposed therebetween both function as free layers. However, when the two free layers are oriented in the same magnetization direction, they do not function as a magnetic field sensor, so it is necessary that there be some measure to bias the two magnetic layers in different directions. This cannot be achieved with a bias using just a conventional hard bias layer, but an extremely complex bias is necessary. Therefore, at present the magneto-resistive effect device with two free layers has not reached the stage of practical use.
On the other hand, strain sensors that use the MR effect have been proposed, and a strain sensor using the MR effect has an area smaller than a conventional strain sensor, and can achieve extremely high sensitivity.
However, in a strain sensor that includes a conventional pinned layer, a spacer layer, and a free layer, there is only one free layer which operates magnetically as a unit (if a free layer is formed in a stacked film, and if there is rotation of magnetization as a unit magnetically, it becomes a single free layer). In this case, if the free layer uses the inverse magnetostrictive effect to detect strain, the magnetostriction coefficient of the free layer is either positive or negative only, so meaningful magnetization rotation only occurs for one of compressive stresses or tensile stresses, so the strain sensor is only capable of detecting one type of strain state. In this case there is the problem that when it is necessary to detect the strain at many points, the total sensitivity is reduced. A strain sensor that is capable of detecting either compressive stresses or tensile stresses is necessary.